This invention relates to modulation of data, tone and audio signals in a duplex radio communications system. More particularly, this invention relates to a simplified circuit architecture for achieving such modulation.
Two-way mobile radio communications systems are growing in popularity, complexity, and cost. Typically, such a system includes a base station and a plurality of mobile stations organized to provide radio communications services among the various mobile stations. Such a system usually allocates one channel to the base station to receive incoming signals from the mobile stations. The base station is allocated a second channel to provide output signals to the mobile stations. Until recently, most of the mobile radio communications included systems designed to handle only voice signals.
However, with the emergence of the mobile digital data terminals, it was found possible to combine the mobile voice terminals with the mobile digital data terminals in a mobile station to provide voice as well as digital data communication capabilities. In addition, it was discovered that purely voice communication systems could be greatly improved by the use of digital data and tone signals (e.g. supervisory audio tones or "SAT") for addressing, handshaking and transmitting status, control or other information among the mobile stations and/or with the base station.
Thus, there has developed a need for a mobile radio station capable of simultaneously handling audio, tone and digital data signals. Many prior art voice/data shared two-way mobile radio systems are capable of operating only in a simplex mode. However, it has been found that demand placed on the channel capacity of the system was compounded when digital data signal transmission is added and thus full duplex channels are sometimes desirable.
In order to provide a full duplex radio communication system capable of handling audio, tone and digital data signals, it is necessary to provide a modulation scheme capable of modulating both audio, tone and digital data signals so that the required data, tone, and audio signals may be received and transmitted at the same time.
Modulation of a single frequency source that is used to provide both transmitter carrier and the first receiver mixer injection frequency signal is somewhat difficult for duplex data. In such a case, both received and transmitted data would be present at the receiver output. However, this mix of data will be corrupted in both phase and amplitude because, at times, the modulated local oscillator injection and the incoming signal will mix to generate an IF signal that consumes more spectrum than the receiver IF bandwidth can pass. Increasing the bandwidth typically is not possible because of external system parameters (e.g. such as FCC channel spacing requirements).
In addition, methods of phase cancellation of the transmitted data at the receiver output are not practical, in part because of typical requirements for complex data recovery techniques. One possible solution to this problem is proposed in U.S. Pat. No. 3,160,812 to Scantlin entitled "Composite Transmission System Utilizing Phase Shift And Amplitude Modulation" issued on Dec. 8, 1964. Scantlin proposes generating a single RF carrier, varying the phase of the carrier as a function of the digital data, amplitude modulating the carrier as a function of the audio signal frequency and radiating the shifted modulated carrier. Besides requiring complex circuitry, and thus increased cost, Scantlin uses separate frequency sources for the transmitter carrier and the receiver mixer injection.
Another possible solution is proposed in U.S. Pat. No. 3,962,638 to Sallis, entitled "Data Transmission Over Voice Bandwidth of FM Radio Channel", issued June 8, 1976. FIG. 2 of Sallis shows a transceiver circuit in which data signals are input to a transmitter oscillator and then to a phase modulator while voice signals are input through a voice input circuit and then to the phase modulator. However, the receiver mixer injection frequency is not derived from either the transmitter oscillator or the phase modulator. Therefore, such a device would require additional synthesizers and thus increase the complexity of the circuit and the cost of the transceiver system.
Yet another possible solution is proposed in U.S. Pat. No. 4,131,849 to Freeburg et al, entitled "Two-Way Mobile Radio Voice/Data Shared Communications System", issued Dec. 26, 1978. Essentially, Freeburg et al permit or inhibit transmission of digital data and voice signals selectively in a way that enables the system to operate in a full duplex mode of operation. However, this circuitry is provided at the base station where separate antennas and separate channels are provided for the received signal and the transmitted signal. Thus, there is no need to provide a single frequency source that is used both for the transmitter carrier and the receiver mixer injection frequencies.
Therefore, there remains a need for a low-cost, simplified modulation scheme for modulating both digital data, tone and audio signals in a full duplex mobile radio transceiver.
Another approach to provide the necessary digital, tone, and audio modulation is to directly drive all the signals simultaneously into a phase modulator following the single synthesizer output which is placed in the transmit branch only. This approach is that proposed for the original Advanced Mobile Phone System Studies. However, the proposal required the synthesizer to operate at one-fourth the desired transmit frequency to achieve the necessary phase deviation at the final transmit frequency. This is due to the fact that a practical phase modulator can only achieve about one-fourth the desired deviation, and frequency multiplication times four gets the deviation where it must be. The latest radio designs which are more simple and lower cost require the synthesizer to operate at the desired transmit frequency and avoid multiplication. The original Bell proposal thus becomes much more difficult because multiple phase modulators in series would be required to achieve the desired phase deviation and this is costly and difficult to do.